Device and method for detecting leakage of filter film

ABSTRACT

An apparatus for detecting leakage through a filtration membrane which comprises a gas-supplying source for supplying a gas to one of two spaces formed by partition with the filtration membrane, a gas-detecting device for measuring the degree of leakage of the gas into the other space, and further preventing the undesirable influence of the supplied gas on the gas-detecting device.

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/JP97/04828 which has an Internationalfiling date of Dec. 25, 1997, which designated the United States ofAmerica.

TECHNICAL FIELD

The present invention relates to apparatus and method for detectingleakage through a filtration membrane which permit highly sensitivedetection of minute defects (e.g. pinholes) present in the filtrationmembrane.

In detail, the present Invention relates to apparatus and method fordetecting leakage through a filtration membrane, in which the influenceof expansion of the filtration membrane caused by pressurization Isfirst excluded and then the flow rate of leaking gas is measured.

BACKGROUND ART

Membrane separation using a filtration membrane is employed in variousfields as a substance separation method which is simple and consumesonly a small amount of energy. The working principle of the membraneseparation is such that) basically, substances are screened byfiltration, depending on the size of pores present In the membrane.Therefore, a uniform desired pore size is important for performancecharacteristics of the membrane.

On the other hand, defects are produced in the filtration membrane insome cases during production or use of the filtration membrane. The mosttypical defect is a defect called pinholes. The pinholes are a smallnumber of pores having a size larger than the desired pore size of thefiltration membrane. If the pinholes are present, substances capable ofpassing through the pinholes are not screened, so that a desiredsubstance in the filtrate is contaminated with the substances whichshould be essentially excluded. Thus, the separation efficiency islowered.

As a method for detecting the pinholes, the minute defect of thefiltration membrane, it is known to use a method which utilizes thesurface tension of a liquid which comprises pressurizing with a gas oneof two spaces formed by partition with the filtration membrane, fillingthe other with the liquid, applying such a pressure that the gas flowsout through the pinholes but not through normal pores, measuring theflow rate of gas leaking from the pinhole portions, and therebyinvestigating the presence of the pinholes.

For example, JP-A-1-307409 discloses three measuring methods, i.e., amethod of directly measuring the flow rate of a gas on the supply side(the starting solution side) after a definite time after the start ofpressurization, a method of measuring the flow rate of a liquid pushedout by the gas, on hollow yarn side (the filtrate side), and a method ofmeasuring the dropping of the liquid level on the hollow yarn side (thefiltrate side)

In addition, JP-A-60-94105 discloses two measuring methods, i.e., amethod of measuring a pressure change due to the leakage of a gas on thebasis of a pressure decrease on the starting solution side, and a methodof directly measuring the leakage of the gas (corresponding to theamount of the gas supplied) by means of a gas flowmeter attached on thestarting solution side.

Further, JP-A-5-157654 discloses a method comprising providing anair-tight gas chamber on the filtrate side and measuring the leakage ofthe gas on the basis of a pressure increase on the side on which the gasleakage and inflow occurs.

However, when the pinholes are small in size or number, the degree ofgas leakage is low, so that it is often difficult to directly measure alow gas flow rate with high precision using a measuring instrument inthe direct measurement method for measuring the gas flow rate.

The method of measuring the amount of a liquid pushed out by the gas andthe method of measuring a pressure change by providing a gas chamber onthe liquid enclosure side permit indirect and high-precision measurementof a low gas flow rate but involve problems. For example, when themembrane expands remarkably due to pressurization or when a precisemeasurement is required in which the expansion of the membrane cannot beneglected: the precision of measurement is decreased because a liquidoutflow or a pressure change, or both are caused by the expansion of themembrane simultaneously with those caused by gas leakage, and theoutflow and/or pressure change caused by the expansion are usuallyundistinguishable from those caused by the gas leakage.

DISCLOSURE OF THE INVENTION

An object of the present invention is to improve an apparatus fordetecting leakage through a filtration membrane and a method for thedetection, i.e., to solve the problem of reduced measurement precisioncaused by the expansion of the filtration membrane by pressurization, bypressurizing with a gas one of two spaces formed by partition with amembrane to a pressure at which the gas does not flow out through normalpores, filling the other space with a liquid, and measuring the flowrate of gas leaking out through pinholes of the filtration membrane bymeasuring the flow rate of liquid pushed out by the gas or by providinga gas chamber on the liquid enclosure side and measuring a pressurechange in the gas chamber.

The present inventor conducted various investigations on the abovesubject and consequently found the following: when there is provided amechanism for first excluding the influence of the expansion of afiltration membrane by pressurization, the flow rate of gas leaking outthrough very small pinholes can be determined efficiently with highprecision, regardless of the expansion of the filtration membrane bypressurization even if the degree of gas leakage is low because thepinholes are small in both size and number. Thus, the present inventionhas been accomplished.

The present inventor also found that the amount of a gas enclosed in thegas chamber can be automatically regulated by a detecting apparatusequipped with the above-mentioned mechanism and the above-mentioneddetecting method, resulting in an increased precision of measurement.

That is, aspects of the present invention are as follows:

(1) An apparatus for detecting leakage through a filtration membranewhich comprises a gas-supplying means for supplying a gas to one of twospaces formed by partition with the filtration membrane, a gas-detectingmeans for measuring the degree of leakage of the gas into the otherspace, and a means for preventing the undesirable influence of thesupplied gas on said gas-detecting means.

(2) An apparatus according to the above item (1), wherein saidgas-detecting means is a pressure-detecting means for measuring thedegree of leakage of the gas in terms of a pressure change.

(3) An apparatus according to the above item (2), wherein the means forpreventing the undesirable influence of the supplied gas on saidgas-detecting means is a three-way valve.

(4) An apparatus according to the above item (2) or (3), wherein saidpressure-detecting means is a micro-differential pressure gauge.

(5) A method for detecting leakage through a filtration membrane whichcomprises pressurizing with a gas one of two spaces formed by partitionwith the filtration membrane to a pressure at which the gas does notflow out through normal pores, filling the other space with an examiningliquid, determining the flow rate of gas leaking out through pinholes ofthe filtration membrane, and thereby investigating the presence of thepinholes of the filtration membrane, wherein the flow rate of theleaking gas is measured after first excluding the influence of theexpansion of the filtration membrane by the pressurization.

(6) A method according to the above item (5), wherein the filtrationmembrane is a virus-separating membrane.

(7) A method according to the above item (6), wherein thevirus-separating membrane is a membrane made of regenerated cellulose.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a schematic view of a typical example of the apparatus fordetecting leakage through a filtration membrane of the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

In the present invention, in order to, for example, detect minutedefects (e.g. pinholes) of a filtration membrane, the filtrationmembrane is pressurized with a gas from one side (gas pressurizationside). A space on the other side (liquid enclosure side) is filled withan examining liquid to wet the filtration membrane therewith. A smallsealed gas space (a gas chamber) is provided on the liquid enclosureside. A pressure detector such as a micro-differential pressure gauge isconnected to the gas chamber, and a slight pressure change caused by gasleaking out through defective portions of the filtration membrane isdetected. In this case, a mechanism for detecting the minute defects isa combination of apparatus and method for detecting the slight pressurechange caused by the leaking gas, though in the present invention, thedetecting mechanism is not limited thereto and also includes, forexample, apparatus and method for measuring the flow rate of liquidpushed out by the gas.

The present invention is characterized in that at the time of thepressurization with a gas in the leakage detection, a mechanism forremoving a flow of the examining liquid caused by the expansion of thefiltration membrane and a pressure increase caused by the flow in theportion filled with the examining liquid, such as, preferably, athree-way valve, is used to discharge the flow caused by the expansionof the filtration membrane and prevent the pressure increase, and thatthe capacity of the sealed gas space is kept constant by automaticallyfilling the space between said filled portion and the three-way valvewith the examining liquid. Because of these characteristics, even aslight leakage can be precisely detected.

In the present invention, the gas-supplying means includes, for example,a gas compressor and a gas cylinder. Usually, a gas obtained from such asource of compressed-gas supply is dehumidified and freed of dust by theuse of a dehumidifier, a filter, etc., and adjusted with a pressureadjuster to a pressure at which said gas does not flow out throughnormal pores, and then the gas is introduced into a space on the gaspressurization side of a filtration membrane to be subjected tomeasurement, through a piping. When the examination is asepticallycarried out, a gas sterilized by means of a filter or the like issupplied.

In the present invention, the gas-detecting means is, for example, apressure-detecting means for measuring the degree of leakage of the gasin terms of a pressure change. As such a means, a pressure gaugesuitable for the gas flow rate range and gas chamber capacity isproperly selected depending on a filtration membrane to be subjected tomeasurement, in addition to a micro-differential pressure gauge.

In the present invention, in order to measure the gas flow rate, theremay be carried out any of the following steps: the detection of a slightpressure change caused by leaking gas, the direct measurement of the gasflow rate, and the measurement of the flow rate of liquid pushed out bythe gas.

In the present invention, the means for preventing the undesirableinfluence of the supplied gas on said gas-detecting means is a mechanismfor excluding the influence of the membrane expansion and/or pressure.As such a means, two two-way valves may be used in addition to three-wayvalves.

The examining liquid used in the present invention, includes water, anaqueous sodium chloride solution, an alcohol solution, and the like.Their use depends on the kind and purpose of the membrane.

FIG. 1 shows an outline of the apparatus for detecting leakage through afiltration membrane of the present invention.

In FIG. 1, numeral 1 denotes a filter container having a filtrationmembrane built-in; numeral 2 denotes the filtration membrane having anedge face closed with a seal 4; numeral 5 denotes a source ofpressurizing-gas supply for pressurizing the filtration membrane with agas; numeral 3 denotes an examining liquid enclosed in a portion not tobe pressurized with the gas in the filtration membrane container 1partitioned with the filtration membrane 2, such as water; numeral 6denotes a three-way valve connected to the liquid-enclosed portion ofthe filtration membrane container 1; numeral 7 denotes an air-tight gaschamber connected to the three-way valve; and numeral 8 denotes adifferential pressure gauge connected to the gas chamber.

The seal 4 is not necessary in some cases, depending on the shape of thefiltration membrane 2. It is sufficient that the filtration membranecontainer 1 is separated into a gas pressurization side A and a liquidenclosure side B by the filtration membrane 2.

Since the detection sensitivity is increased with, for example, adecrease of the capacity of the gas chamber 7, the gas chamber 7 alsoneed not be positively attached if the capacity of an air-tightconnecting piping 10 itself between the three-way valve 6 and the gaschamber 7 in FIG. 1 is sufficient as the capacity of a gas chamber.

The differential pressure gauge may be an ordinary pressure gauge,depending on the flow rate of leaking gas.

The filtration membrane 2 subjected to leakage detection by the methodof the present invention is not particularly limited and may be anyfiltration membrane. The filtration membrane 2 includes, for example,micro-filtration membranes (microfilters, MF), reverse osmosismembranes, ultrafiltration membranes (UF), and virus-separatingmembranes. As to the shape of the membrane, any membrane, such as aplain membrane or a hollow-yarn membrane, can be examined in the presentinvention, irrespective of its shape.

Particularly in a regenerated-cellulose microporous membrane composed ofa hollow-yarn membrane having a high virus-removing capacity and a highprotein permeability, high-precision detection of pinholes is desired asa virus-separating membrane because of the high removing capacityrequired of the microporous membrane. The apparatus and method fordetecting leakage of the present invention realizes such high-precisiondetection easily and efficiently.

A more specific example of the method for detecting leakage through afiltration membrane of the present invention is described below bytaking the case of the apparatus for detecting leakage shown in FIG. 1.

(1) Preparatory stage

First, the three-way valve 6 is operated so that an outflow from thefiltration membrane container 1 may be discharged outside the system asit is.

The space outside the filtration membrane 2 in the filtration membranecontainer 1 is filled with water as the examining liquid 3 formeasurement. In this case, the space may be filled with water afterdetachment of the source of pressurizing-gas supply (on the gaspressurization side A) while filtering the examining liquid (water), orthe space may be filled with water through an opened valve 9 over thefiltration membrane container 1.

(2) Preliminary pressurization (the exclusion of the influence ofmembrane expansion)

The three-way valve 6 is kept in such a state that a gas outflow fromthe filtration membrane container 1 is discharged outside the system asit is.

The space inside the filtration membrane 2 is pressurized toninety-eight kPa (one kg/cm²) with a gas (air). When water is used asthe examining liquid and air is used as the pressurizing gas, thepressurization pressure of ninety-eight kPa is a pressure at which whenthe contact angle is taken as one, the gas passes through pores with adiameter of about three microns or more but not through pores with adiameter of less than three microns. The filtration membrane examinedfor leakage in the present invention is, for example, a micro-filtrationmembrane (a microfilter, MF), reverse osmosis membrane, ultrafiltrationmembrane (UF), or virus-separating membrane. Since all the normal poresof any of these membrane have a diameter of less than three microns, airdoes not pass through the normal pores.

In this case, when the filtration membrane 2 is expanded or pinholeswith a diameter of three microns or more are present, the examiningliquid (water) and the pressurizing gas (air) are discharged outside thesystem through the three-way valve 6 by the leakage the pressurizing gas(air).

The expansion of the filtration membrane 2 is mostly an elastic changeand terminates in a short time of one hundred eighty seconds or less.

(3) Pressure measurement

After a previously estimated expansion time of the membrane (one hundredeighty seconds), the three-way valve 6 is operated at the pressurizedstate manually or automatically with a timer so that the filtercontainer 1 may communicate with the gas chamber 7.

In this case, if the degree of expansion (volume) of the filtrationmembrane 2 remarkably exceeds the degree of leakage (volume) of the gas(air), the space between the filtration membrane container 1 and thethree-way valve 6 is automatically filled with the examining liquid 3(water), and the capacity of the gas chamber 7 is always kept constantas the total capacity of the gas chamber itself and the connectingpiping 10 between the gas chamber and the three-way valve, so that theextent of errors in the pressure change measurement produced by a changein capacity of the gas chamber is reduced, resulting in an improvedprecision of measurement.

A pressure increase in a definite time thirty seconds) after theoperation of the three-way valve is measured with the differentialpressure gauge 8.

(4) Judgement

The pressure measured may be used as it is as a measured value or may beconverted to a flow rate on the basis of the capacity of the gas chamber7.

Even in the case of a normal filtration membrane free from leakage,measured values are usually not zero owing to, for example, gasdiffusion. Therefore, the average and the scatter of measured values aredetermined, and the thus obtained value is compared with a valuemeasured for a filtration membrane examined for leakage, whereby theoccurence of leakage through this filtration membrane can be determined.

EXAMPLE 1

By the use of an apparatus having the structure shown in FIG. 1, resultsobtained by carrying out preliminary pressurization (the exclusion ofthe influence of membrane expansion) were compared with those obtainedwithout preliminary pressurization, by using each of filters havingpinholes and filters having no pinholes. As the filters subjected tomeasurement, there were used filters with an average pore diameter offifteen nm and a membrane area of one m² (PLANOVA, a registered tradename, mfd. by Asahi Kasei Kogyo K.K.).

As the measurement conditions, the pressurization time and the pressuremeasurement time were one hundred eighty seconds and thirty seconds,respectively, in the case of carrying out the preliminary pressurizationand were three seconds and thirty seconds, respectively, in the case ofnot carrying out preliminary pressurization. In both cases, the capacityof the gas chamber was adjusted to sixteen cc (ml).

As to a measuring procedure, the three-way valve 6 (mfd. by COSMOINSTRUMENT CO., LTD.) was operated at first in a preparatory stage sothat an outflow from the filtration membrane container 1 might bedischarged outside the system as it was. Then, water is introduced intothe space outside the filtration membrane in the filtration membranecontainer 1 from the opened valve 9 over the filtration membranecontainer 1 to fill the space with water. Subsequently, the space insidethe filtration membrane 2 was pressurized to ninety-eight kPa (onekg/cm²) with a gas (air) while keeping the three-way valve 6 in such astate that the outflow from the filtration membrane container 1 wasdischarged outside the system as it was. When the preliminarypressurization was carried out (when the influence of membrane expansionwas excluded), this condition was maintained for one hundred eightyseconds. When no preliminary pressurization was carried out, thecondition was maintained for three seconds. After the maintenance ofsaid condition, the three-way valve was operated so that the filtercontainer 1 might communicate with the gas chamber 7, and pressuremeasurement was carried out.

A pressure increase value thirty seconds after the operation of thethree-way valve was measured with the differential pressure gauge 8(mfd. by COSMO KEIKI K.K.).

As the filters having pinholes, there were used two filters, i.e., afilter having relatively remarkable pinholes and a filter havingrelatively slight pinholes. For both filters having pinholes, theproduction of bubbles through the filtration membrane 2 could be foundby visual observation when in FIG. 1, the space on the liquid enclosureside B was filled with water and the space on the pressurization side Awas pressurized to ninety-eight kPa with air. In this case, a largeamount of bubbles were produced through the filter having relativelyremarkable pinholes, and bubbles were also produced through the filterhaving relatively slight pinholes. On the other hand, in the case of thefilters having no pinholes, no bubbles were produced at all when in FIG.1, the space on the liquid enclosure side B was filled with water andthe space on the pressurization side A was pressurized to ninety-eightkPa with air.

Table 1 shows pressure change values obtained in a measurement time ofthirty seconds in the case of not carrying out preliminarypressurization (pressurization time: three seconds), and Table 2 showspressure change values obtained in a measurement time of thrity secondsin the case of carrying out the preliminary pressurization(pressurization time: one hundred eighty seconds). In the tables, thesymbols for the filters F_(A), F_(B), and F_(C) denote the filtershaving no pinholes, F_(D) denotes the filter having relativelyremarkable pinholes, and FE denotes the filter having slight pinholes.In Table 2, for the filters having pinholes F_(D) and F_(E), thepressure change values were calculated according to the rule of threebecause they exceeded the upper limit of measurement in about 1.5seconds.

                  TABLE 1                                                         ______________________________________                                        When no preliminary pressurization was carried                                out.                                                                                   F.sub.A                                                                             F.sub.B F.sub.C  F.sub.D                                                                              F.sub.E                                ______________________________________                                        Pressure change                                                                          4900    4508    3038   28028  7644                                 value (Pa)                                                                    ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        When preliminary pressurization was carried out.                                       F.sub.A                                                                             F.sub.B F.sub.C F.sub.D                                                                              F.sub.E                                 ______________________________________                                        Pressure change                                                                          2793    2999    3067  19600  19600                                 value (Pa)                                                                    ______________________________________                                    

In both Table 1 and Table 2, the pressure change was greater in the caseof the two filters having pinholes F_(D) and F_(E) than in the case ofthe three filters having no pinholes F_(A), F_(B), and F_(C). However,there was a large difference between the measurement sensitivityattained when no preliminary pressurization was carried out and thatattained when the preliminary pressurization was carried out. Table 3shows the difference between a mean value obtained for filters having nopinholes and each measured value for the filter having pinholes.

                  TABLE 3                                                         ______________________________________                                                                      Preliminary                                                        No preliminary                                                                           pressurization                                  Statistic          pressurization                                                                           (180 sec.)                                      ______________________________________                                        No pinholes                                                                           Mean of pressure change                                                                      4149        2953                                               (Pa)                                                                          Standard deviation of                                                                        982         143                                                pressure change (Pa)                                                  Pinholes                                                                              Measured value for                                                                           7644       19600                                       were present                                                                          filter F.sub.E                                                        Difference between mean value for filt-                                                          3495       16647                                           ers having no pinholes and measured                                           value for filter having pinholes (Pa)                                         (Difference from mean value for filters                                                          3.6         116                                            having no pinholes)/standard deviation                                        ______________________________________                                    

When the difference between a mean value obtained for the filters havingno pinholes and a pressure change value measured for the filter havingpinholes is four to five times or more as large as the standarddeviation, the presence of pinholes can be conventionally determined.However, the presence of pinholes in the filter F_(E) could not beclearly determined without preliminary pressurization because saiddifference is small. On the other hand, when the preliminarypressurization for one hundred eighty seconds was carried out in thecase of one and the same filter F_(E), the employment of the mechanismfor removing a pressure change due to the membrane expansion results ina sufficiently large difference between a mean value obtained for thefilters having no pinholes and a pressure change value measured for thefilter having pinholes, so that the presence of pinholes in the filterF_(E) could be sufficiently determined. As a result of carrying out thepreliminary pressurization for one hundred eighty seconds to exclude theinfluence of the membrane expansion, both the absolute values ofpressure change and the scatter of the values expressed in terms of astandard deviation were reduced. Consequently, the difference betweenthe mean value obtained for the filters having no pinholes and thepressure change value measured for the filter having pinholes waswidened, resulting in an increased detection sensitivity.

According to the present invention, the influence of the expansion of afiltration membrane by pressurization is previously excluded by means ofa three-way valve or the like, so that a slight gas leakage through afiltration membrane having minute defects such as pinholes can bedetected with high precision even if the filtration membrane isexpanded. Therefore, even very small pinholes or a small number ofpinholes of the filtration membrane can be detected with highsensitivity.

We claim:
 1. An apparatus for detecting leakage through a filtrationmembrane which comprises a gas-supplying means for supplying a gas toone of two spaces formed by partition with the filtration membrane, agas-detecting means for measuring the degree of leakage of the gas intothe other space, and a means for preventing the undesirable influence ofthe supplied gas on said gas-detecting means.
 2. An apparatus accordingto claim 1, wherein said gas-detecting means is a pressure-detectingmeans for measuring the degree of leakage of the gas in terms of apressure change.
 3. An apparatus according to claim 2, wherein the meansfor preventing the undesirable influence of the supplied gas on saidgas-detecting means is a three-way valve.
 4. An apparatus according toclaim 2 or 3, wherein said pressure-detecting means is amicro-differential pressure gauge.
 5. A method for detecting leakagethrough a filtration membrane which comprises pressurizing with a gasone of two spaces formed by partition with the filtration membrane to apressure at which said gas does not flow out through normal pores,filling the other space with an examining liquid, determining the flowrate of gas leaking out through pinholes of the filtration membrane, andthereby investigating the presence of the pinholes of the filtrationmembrane, wherein the flow rate of the leaking gas is measured afterfirst excluding the influence of the expansion of the filtrationmembrane by the pressurization.
 6. A method according to claim 5,wherein the filtration membrane is a virus-separating membrane.
 7. Amethod according to claim 6, wherein the virus-separating membrane is amembrane made of regenerated cellulose.